The accumulation of valuable nutrients in grains is dependent on a number of factors, including biotic and abiotic stress, and the effects of herbicides. The aim of this study was to determine the effect of a metazachlor and clomazone mixture and 2-methyl-4-chlorophenoxyacetic acid (MCPA) on the chemical composition of the neutral detergent fibre, lipids, protein and ash content of common buckwheat (Fagopyrum esculentum Moench) seeds. A field experiment was conducted in 2013-2015. The mixture containing metazachlor (750 g ha -1 ) and clomazone (96 g ha -1 ) applied one day after buckwheat sowing proved to be phytotoxic and caused leaf chlorosis. These symptoms persisted until the end of the flowering stage and gradually disappeared during the seed formation stage. MCPA applied at the stage of two true leaves at a dose of 750 g ha -1 showed a higher phytotoxic effect, manifested by deformation and discoloration of the buckwheat stems. MCPA as well as mixture containing metazachlor and clomazone significantly increased the content of neutral detergent fibre (NDF), its fractions (hemicellulose, cellulose and lignin) and proteins compared with the control treatment. However, they caused a significant decrease in the ash content. Compared to the untreated control buckwheat seeds, application of MCPA caused an increase in the content of the following compounds: neutral detergent fibre (1.2%), hemicellulose (0.8%), cellulose (11.1%), lignin (0.6%) and protein (2.03%); mixture metazachlor and clomazone increase: neutral detergent fibre (0.8%), hemicellulose (0.5%), cellulose (11.3%), lignin (0.43%) and protein (0.60%). Each of the surveyed phytotoxic substances caused stress to buckwheat plants, with MCPA applied in the stage of two true leaves at a dose of 750 g ha -1 being the most aggressive stressor that caused deformation of the buckwheat plants, decreased yield (0.68 t ha -1 ) and increased protein content (2.03%) in the buckwheat grains.